The present invention relates to an underwater detection system for displaying underwater conditions on an indicator comprised of, for example, a monochrome cathode ray tube with various degrees of brightness, depending on the amplitude of echo signals resulting from an ultrasonic search signal radiated into the water. Particularly, it relates to a detection system for displaying underwater conditions with two groups of different degrees of brightness. One group of various degrees of brightness has a relationship with the amplitude of an echo signal in such a way that degrees of brightness are in direct proportion to the amplitude of an echo signal. The other group of different degrees of brightness has another relationship with the amplitude of an echo signal.
For example, with one group of various degrees of brightness, the seabed produced strong echo signals is displayed in "white", and a large school of fish produced a medium echo signal is displayed in "light gray", and a fish school, small in size or staying distant from the water surface, producing a weak echo signal is displayed in "dark gray", and the rest of the indicating surface of the indicator is displayed in "black". With the other group of various degrees of brightness, the seabed, a large fish school, a small fish school and no objects are displayed in "dark gray", "white", "light gray" and "black" respectively.
Conventional underwater detection systems have displayed underwater conditions with one group of various degrees of brightness, the degrees of brightness being in direct proportion to the amplitude of an echo signal. These detection systems display the seabed with maximum brightness, i.e., in "white", a fish school staying close to the seabed in "dark gray" and no objects in "dark gray". Therefore, a fish school staying close to the seabed is hardly distinguished from the seabed, since the fish school displayed with lesser brightness is affected by the seabed which is displayed with maximum brightness.